In the above-cited copending patent application there is described a system for descaling and leveling a metal strip wherein the strip is passed through a pair of succeeding pressing assemblies. In the upstream assembly the strip is subjected to a relatively low degree of longitudinal elongation, equal to the maximum descaling stretch (MDS). At the downstream assembly the extent of stretch is considerably greater so as completely to eliminate lateral bowing of the strip. Thus in accordance with my earlier invention the strip in a single pass is both completely descaled and perfectly leveled, both of these operations being carried out at levels which correspond to the ideal stretch for that operation. The maximum descaling stretch in the upstream device is between 0.5% and 1.3% and the extent of stretch in the downstream assembly is between 2% and 2.5%.
It is known to drive a metal-strip stretching device by connecting all of the rollers through a three-part differential gear assembly to a common drive shaft driven by a single large-capacity drive motor. One part of each differential gear assembly is connected to the roller, another part is connected to the drive shaft, and a third part is connected to a so-called compensating motor. The function of these compensating motors is to create the desired tension in the metal strip, as each compensating motor is driven at a speed that establishes the necessary stretch between it and the roller immediately upstream.
This system has the disadvantage that variations in the thickness of the band or in other parameters of the process create so called transverse shatter marks in the strip. This is due to the fact that the compensating motors are operated at a single ideal speed set for a particular thickness of strip, speed of displacement, and the like.
It has been suggested to overcome this by using motors which deliver constant torque. Such a system has the disadvantage that the torque is affected by a multiplicity of different process parameters and can rarely be controlled with sufficient precision. Since transmission ratios usually in the neighborhood of 1:250 or more are used it is possible for very small variations in the output torque to have a relatively large effect in the end product. Thus it is possible for a simple heating of the motor, causing a decrease in viscosity of the fluid therein, to have a relatively great effect on the quality of the finished product.